1. Field of the Invention
The present invention relates to control systems for hydraulically actuated relief valves and more particularly to control systems for hydraulically actuated relief valves which are connected to the suction line of a hydraulic dredge.
2. General Background
A hydraulic dredge, of the type used to dredge water bottoms, generally includes a barge, a pump, a suction line and either a boom or a crane. The pump is generally a centrifugal pump having a suction inlet and a discharge outlet. The suction inlet of the centrifugal pump is connected to one end of the suction line. The intake end of the suction line is controlled by the boom or crane, which directs the depth and location of the intake end of the suction line. The pump discharge outlet is connected to a discharge line.
The dredging operation consists generally of positioning the intake end of the suction line near or on the material to be dredged while the pump is operating. The material to be dredged is drawn up the suction line, through the pump, ejected from the discharge outlet of the pump, through the discharge line, and deposited onto the barge.
During dredging, three events may occur which can halt or impair the dredging operation. The first event is blockage of the intake end of the suction line by the dredged material. This intake blockage creates an elevated vacuum level in the suction line. The second event is discharge line blockage caused by dredged material precipitating out from the water and building up in the discharge line. This discharge line blockage creates high pressure at the discharge outlet. Either condition, a high vacuum level or a high pressure level, can cause damage to the pump.
To prevent damage to the pump during absolute blockages and to allow the pump time to work through partial blockages, a relief valve is usually installed in the suction line intermediate the intake end and the centrifugal pump. The relief valve allows water to enter the suction line from a location other than the fully or partially blocked intake end. The relief valve is generally actuated by hydraulic pressure controlled by a control system which monitors the vacuum level in the suction line and the pressure level in the discharge line. When either the vacuum level or the pressure level reaches a predetermined alarm threshold, the control system opens the relief valve allowing water to enter the intake line. The control system closes the relief valve once both the vacuum level and pressure level are below the predetermined alarm threshold.
The third event which may halt or impair dredging operations is failure of the hydraulic power unit due to electrical power interruptions. When the hydraulic power unit fails, hydraulic pressure in the lines connected to the relief valve actuator is no longer sufficient to position the valve. This means the relief valve is stuck in the position in which it happened to be when the hydraulic pump failure occurred. If the relief valve is in the closed position when the hydraulic pump fails, either the vacuum level in the suction line or the pressure level in the discharge line may reach a level which are dangerously high. This condition can significantly increase the risk of damage to the dredge. What is needed is a device, included in the control system, which will allow the operator of the dredge to position the relief valve in a desired position after a hydraulic pump failure has occurred.